Question

A detention basin is designed to trap all particles larger than 0.05 mm in size with a specific gravity of 2.65 at 20${}^o$C temperature. The effective depth of the tank is 4 m and the length of the tank is 50 m. The horizontal flow velocity to be recommended for effective removal of particles is

• A. 3.8 cm/sec
• B. 3.5 m/sec
• C. 2.8 mm/sec
• D. 2.8 cm/sec

Answer 1

The correct option is D 2.8 cm/sec

$d\le 0.1mm,$ L = 50 m, H = 4 m

Hazen's formula for settling velocity $V_s$,

$V_s=418(s-1)d^2\frac{(3T+70)}{100}$

$=418(2.65-1)\times (0.05{)}^2\frac{(3\times 20+70)}{100}$

$V_s=2.24$ mm/sec = 0.224 cm/sec

Conditions for effective removal

$\left(i\right)V_{si}>V_0$

$\left(ii\right)H/V_s\le L/V_H$

$\frac{V_H}{V_s}=\frac{L}{H}\Rightarrow V_H=V_s\times \frac{L}{H}$

$=\frac{0.224\times 5000}{400}=2.8cm/sec$